This invention relates to a plating method for steel sheet. More particularly, it relates to an electroplating method of hot-galvanized steel sheet. It also relates to a plating line in which a steel sheet is continuously subjected to hot-galvanizing and then electroplating. The resulting plated steel sheet has an electroplated top coating with excellent covering power and adhesion to the underlying hot-galvanized coating.
In the automotive and construction industries, there is always a great demand for materials having good corrosion resistance and a long life span. In particular, the corrosion resistance demanded of rust-preventive steel sheets for automobile bodies has become extreme.
In order to meet these demands, various new types of electroplated steel sheets have been proposed, such as steel sheets electroplated with a Zn-Ni, Zn-Fe, or Zn-Mn alloy, steel sheets hot-dip plated with a Zn-Fe, Zn-Al-Si, or Zn-Al-Mg alloy. Steel sheet having multiple plated layers in which the top layer is an Fe-rich (Fe.gtoreq.60%) Fe-Zn alloy plated coating has also been developed with the intention to improve the coatability of the plated steel sheet by cationic electrodeposition performed thereon and to increase the adhesion of the electrodeposited coating in water (see Japanese Published Unexamined Patent Application No. 56-133488).
Steel sheet with a plurality of layers of plated coating (hereinafter referred to as multi-layer plated steel sheet) is highly suitable for use in automobiles and as a construction material not only on account of its coatability but also because of its excellent press forming characteristics (sliding properties), weldability, and various other properties.
The multi-layer electroplating that have been proposed in the prior art include a Zn-Ni/Fe or Fe-Zn coating (Japanese Published Examined Patent Application No. 60-57518), a Zn-Ni/Zn or Zn-Ni or Zn-Fe/Cr(Cr-oxide) coating (Japanese Published Unexamined Patent Application No. 60-197893), a Zn-Mn/Zn-Fe coating (Japanese Published Unexamined Patent Application No. 58-42787), and a Zn or Zn alloy/minute particle-dispersed Zn or Zn alloy coating (Japanese Published Unexamined Patent Application No. 62-230999).
Recently, it has also been proposed to perform electroplating on an alloyed hot-galvanized steel sheet (Japanese Published Unexamined Patent Applications Nos. 56-133488 and 61-253397).
When forming multi-layer electroplated coating using a single electroplating line, normally, plating baths for different types of coatings are arranged in series along the line. Equipment for dip water rinsing and, if necessary, equipment for rinsing with hot water or with brushes is installed between successive baths. However, no treatment other than rinsing or scrubbing is performed on the steel sheet as it is passed from one bath to another.
Similarly, when a steel sheet is hot-galvanized and then electroplated in a continuous process, equipment for continuous electroplating is simply connected in series with equipment for continuous hot-galvanizing, and no special treatments are performed on the steel sheet as it travels between the two sets of equipment.
For example, Japanese Published Unexamined Patent Application No. 60-224791 discloses a continuous plating apparatus in which a pretreatment apparatus, a hot-galvanizing bath, an alloying furnace, and an electroplating apparatus are connected in series. A skin-pass rolling mill and, if necessary, a water rinse tank may be disposed between the hot-galvanizing bath and the electroplating apparatus.
Japanese Published Unexamined Patent Application No. 62-17200 discloses a continuous one-sided plating apparatus in which a pretreatment apparatus, a hot-galvanizing bath for plating one side of a steel sheet, an alloying furnace, a cleaning apparatus for cleaning the unplated side of the sheet, and an electroplating apparatus are connected in series.
In these continuous plating apparatuses, no special chemical treatment is performed on the hot-dipped coating of the sheet before electroplating.
However, the present inventors' research has shown that when two different processes, such as hot-galvanizing and electroplating, are arranged in sequence, the following problems occur.
(1) If continuous electroplating is performed after hot-galvanizing of a steel sheet, electroplated coatings such as a Fe or Fe-based alloy (Fe-Zn, etc.), Cr(Cr-oxide), Ni, and Zn-Ni alloy coatings have poor adhesion to the galvanized coating, and these coatings tend to readily peel off either while the coated sheet is still flat or after it has been subjected to working (bending, drawing, etc.).
(2) If a hot-galvanized steel sheet is heated to perform alloying of the galvanized coating prior to electroplating, the resulting alloyed galvanized coating has microscopic surface irregularities, i.e., bumps and depressions, which are inherent in an alloyed galvanized steel sheet (usually called GA steel sheet). The irregularities include those which are caused by the crystalline form of the Zn-Fe alloy and microscopic depressions which are formed during alloying. They generally have a size of 3-20 micrometers.
Such microscopic surface irregularities, and particularly the depressions, cannot be adequately covered by the overlaid electroplated coating. When the electroplated coating is one such as an Fe coating which is intended to increase the coatability of the plated steel sheet by cationic electrodeposition, the electroplated coating cannot adequately perform its intended function.
The covering power of an electroplated coating with respect to microscopic irregularities will hereunder be referred to as its microcovering power.
Thus, it is not possible to achieve a hot-galvanized electroplated steel sheet of high quality simply by connecting a continuous hot-galvanizing apparatus and a continuous electroplating apparatus in sequence.